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Cross Shaped "Star Filter" Aberation

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#1 TinPusher

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Posted 12 July 2018 - 12:18 AM

So I got my new Baader Classic Orthos. Wow! Big improvement over my Meade China made 4000 series. I am not so sure about the Baader Q Barlow. Need to test it further, but it certainly seemed no better than the Meade 126 cheap Barlow. I did not try it in the mode where you remove the bottom lens and attach it to the eyepice directly.

 

I noticed tonight that when looking at Jupiter I had a cross shaped (classic four point photography star filter) aberration on Jupiter. It was not based on the rotation of the eyepiece. It occurred in all the shorter eyepieces (18mm down). The seeing here in St Louis was very good.

 

I am wondering if it was the spider vanes, as I think it was aligned with them. I tried moving my hands around them to see if I could vary it, but I didn't see much change (wasn't sure if it was my neighbor's lights hitting the spider vanes, but I don't think so.) I need to make a aperture disc for this scope, but is this something that happens with bright bodies hitting the spider vanes? I recently collimated the scope, and I think I got it pretty nice, but I am no expert there. Was thinking a laser collimator would be a good investment, as this was with a Cheshire.

 

The scope is a LightBridge 12" Dob that is over 5 years old. It is one of the earlier models with silver truss poles (need to paint those).

 

The Baader Q-Turret does not seem to work with the focuser on my LightBridge. My focuser is the older one without the fine focus (grrr). With the turret I could not get the focus tube short enough to get things in focus.

 

Any thoughts? Thanks.


Edited by TinPusher, 12 July 2018 - 12:24 AM.


#2 Tom Masterson

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Posted 12 July 2018 - 12:58 AM

Sounds like typical diffraction spikes caused by the spider. Perfectly normal, not an aberration.  The brighter the object, the brighter the spikes.

 

How close are you to getting focus with the turret? Is your primary mirror cell mounted on springs? If you are very close sometimes you can move the mirror forward a little by loosening the collimation screws a few turns allowing the springs to expand. If you are a ways away it may take some surgery on the scope such as moving the primary mirror cell forward or purchasing a lower profile focuser.



#3 TinPusher

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Posted 12 July 2018 - 01:35 AM

Thanks for the reply, and thanks for the reassurance. I figured it was from the spider vanes, but wanted to make sure this was normal. I guess it must be diffraction.

 

I guess the easiest solution is the small aperture mask?

 

The mirror is on a type of springs (more like leaf springs). The focus was fairly far from sharp. I think it was probably a full turn of the knob or so away. I will have to explore further, but I was a little bummed. The turret seemed like a neat idea. The directions claim the BCOs are parafocal in the turret, which would be cool. I did not find them to be precisely parafocal when inserted into the normal eyepiece holder.

 

Need to do some more testing of the Barlow, but that item did not impress me. The eyepieces themselves where very impressive. Even the 32mm Plossl is a step up from the Meade 4000 (China) series.

 

I was also testing out a new Orion branded 80A filter for Jupiter. I am hoping it was dirty (I forgot to clean it before I went out-- I often clean brand new optics) because it had significant image degradation (slight softening and soft blooming around everything). Sure, it slightly increased the contrast on Jupiter's streaks and the red spot, but it was like looking through a piece of Saran Wrap (OK, not that bad, but not premium optical quality). Will try again now that I have cleaned it thoroughly. I looked clean, but I know a lot of times filters get dirty from the case they come in, either from contact with the foam, or from outgasing of the foam. Given the price I can figure Orion is likely not super high grade. Honestly, the BCOs are so contrasty, that with good seeing Jupiter looks pretty darned contrasty as is with my 12" Dob.



#4 Jon Isaacs

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Posted 12 July 2018 - 07:22 AM

I guess the easiest solution is the small aperture mask?

 

An aperture mask will eliminate the diffraction spikes but you will be left with the resolution and contrast of a 4.5 inch scope.  Another solution is a curved spider, it spreads out the diffraction spikes so you do not see them.  There is a slight loss of contrast due to their larger area and then there's the issue of secondary stability.  

 

I just accept the spikes and enjoy the views.

 

Jon



#5 csrlice12

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Posted 12 July 2018 - 03:49 PM

I'm finding I prefer refractors for planets, the dob for most everything else.

 

P.S.  I really hate autocorrect, cause it ain't always autoright.



#6 Peter Besenbruch

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Posted 13 July 2018 - 08:05 PM

So I got my new Baader Classic Orthos. Wow! Big improvement over my Meade China made 4000 series. I am not so sure about the Baader Q Barlow. Need to test it further, but it certainly seemed no better than the Meade 126 cheap Barlow. I did not try it in the mode where you remove the bottom lens and attach it to the eyepice directly.

 

I noticed tonight that when looking at Jupiter I had a cross shaped (classic four point photography star filter) aberration on Jupiter. It was not based on the rotation of the eyepiece. It occurred in all the shorter eyepieces (18mm down). The seeing here in St Louis was very good.

 

I am wondering if it was the spider vanes, as I think it was aligned with them. I tried moving my hands around them to see if I could vary it, but I didn't see much change (wasn't sure if it was my neighbor's lights hitting the spider vanes, but I don't think so.) I need to make a aperture disc for this scope, but is this something that happens with bright bodies hitting the spider vanes? I recently collimated the scope, and I think I got it pretty nice, but I am no expert there. Was thinking a laser collimator would be a good investment, as this was with a Cheshire.

 

The scope is a LightBridge 12" Dob that is over 5 years old. It is one of the earlier models with silver truss poles (need to paint those).

 

The Baader Q-Turret does not seem to work with the focuser on my LightBridge. My focuser is the older one without the fine focus (grrr). With the turret I could not get the focus tube short enough to get things in focus.

The Turret is meant for refractors with some in-focus capability, and SCTs. Newtonians need not apply.

 

I never liked the Q-Barlow, either. Consider a longer Barlow design with your scope.

 

Your spider is causing the spikes on Jupiter. Standard Newtonian effect. I wouldn't worry about it.

 

I own the 32mm, the 18mm, and the 10mm. I find them very good, though I don't know how well they work at f5.




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